diff --git a/src/presentation/http/http-api.test.ts b/src/presentation/http/http-api.test.ts
new file mode 100644
index 00000000..b3e82fb1
--- /dev/null
+++ b/src/presentation/http/http-api.test.ts
@@ -0,0 +1,79 @@
+import { describe, test, expect } from 'vitest';
+
+describe('HTTP API Error Handler', () => {
+  describe('JSON Parse Error Handler', () => {
+    test('Returns 400 when request body contains not complete JSON', async () => {
+      const response = await global.api?.fakeRequest({
+        method: 'POST',
+        url: '/join/test-hash1',
+        headers: {
+          // eslint-disable-next-line @typescript-eslint/naming-convention
+          'Content-Type': 'application/json',
+        },
+        body: '{invalid json',
+      });
+
+      expect(response?.statusCode).toBe(400);
+
+      const body = await response?.json();
+
+      expect(body).toStrictEqual({
+        message: 'Invalid JSON in request body',
+      });
+    });
+
+    test('Returns 400 when request body contains malformed JSON', async () => {
+      const response = await global.api?.fakeRequest({
+        method: 'POST',
+        url: '/join/test-hash1',
+        headers: {
+          // eslint-disable-next-line @typescript-eslint/naming-convention
+          'Content-Type': 'application/json',
+        },
+        body: '{"key": "value",}',
+      });
+
+      expect(response?.statusCode).toBe(400);
+
+      const body = await response?.json();
+
+      expect(body).toStrictEqual({
+        message: 'Invalid JSON in request body',
+      });
+    });
+
+    test('Returns 400 when JSON body is empty', async () => {
+      const response = await global.api?.fakeRequest({
+        method: 'POST',
+        url: '/join/test-hash1',
+        headers: {
+          // eslint-disable-next-line @typescript-eslint/naming-convention
+          'Content-Type': 'application/json',
+        },
+        body: '',
+      });
+
+      expect(response?.statusCode).toBe(400);
+
+      const body = await response?.json();
+
+      expect(body).toStrictEqual({
+        message: 'Invalid JSON in request body',
+      });
+    });
+
+    test('Does not return 400 for valid JSON', async () => {
+      const response = await global.api?.fakeRequest({
+        method: 'POST',
+        url: '/join/test-hash1',
+        headers: {
+          // eslint-disable-next-line @typescript-eslint/naming-convention
+          'Content-Type': 'application/json',
+        },
+        body: '{"key": "value"}',
+      });
+
+      expect(response?.statusCode).not.toBe(400);
+    });
+  });
+});
diff --git a/src/presentation/http/http-api.ts b/src/presentation/http/http-api.ts
index 6e5cb37a..ffc41d8e 100644
--- a/src/presentation/http/http-api.ts
+++ b/src/presentation/http/http-api.ts
@@ -34,6 +34,11 @@ import UploadRouter from './router/upload.js';
 import { ajvFilePlugin } from '@fastify/multipart';
 import { UploadSchema } from './schema/Upload.js';
 import { NoteHierarchySchema } from './schema/NoteHierarchy.js';
+import { StatusCodes } from 'http-status-codes';
+
+interface FastifyError extends Error {
+  code: string;
+}
 
 const appServerLogger = getLogger('appServer');
 
@@ -372,6 +377,21 @@ export default class HttpApi implements Api {
 
         return;
       }
+      /**
+       * JSON parse errors (invalid request body)
+       * Errors can be either SyntaxError or FastifyError.
+       */
+      if ((error instanceof SyntaxError && error.message.includes('JSON'))
+        || ((error as FastifyError).code?.startsWith('FST_ERR_CTP_') ?? false)) {
+        this.log.warn({ reqId: request.id }, 'Invalid JSON in request body');
+
+        return reply
+          .code(StatusCodes.BAD_REQUEST)
+          .type('application/json')
+          .send({
+            message: 'Invalid JSON in request body',
+          });
+      }
       /**
        * If error is not a domain error, we route it to the default error handler
        */